Weird shading effects when rendering at long distance
If you change the start clip distance on the camera to a higher number the black edges disappear.

  



The reason for this has to do with how blender approximates the calculations for the Z-Depth within the clipping distance.

It's very well explained here: http://blendermama.com/black-faces-error-from-camera-clipping.html

In this other link: https://discoverylinux.wordpress.com/2011/10/08/guida-eliminare-il-flickering-in-blender-2-5/ the author recommends keeping the start and end clip distances to contain only the geometry in the scene for better precision.